Commonly used inductive components feature a magnetic core made of a soft magnetic material with one or more wire windings wound around it. The magnetic core can be ring-shaped or have any other form and can be made of ferrite or of an amorphous or nanocrystalline material. Magnetic cores can also be formed of wound tape and, in this case, they are known as tape wound cores.
Inductive components include, for example, current-compensated (common mode) chokes. In order to manufacture current-compensated chokes that have a high level of impedance and yet a small number of windings, tape wound cores of amorphous nanocrystalline tape are preferred. Such tape wound cores make it possible to combine high permeability (for example μ>100.000) with good high frequency behavior while achieving a volume-optimized choke design with low ohmic losses.
There are, however, no efficient adjustment methods for magnetic cores made of wound tape. Adjustment is necessary because the actual inductivity value of the inductive components generally differs greatly from the target value, depending on the manufacturer. Adjustment methods used for other kinds of magnetic cores or other magnetic materials cannot easily be applied to tape wound cores. For example, a fine-tuning of the inductivity tolerance by grinding the core or by adjusting a screw core, which is common practice with certain kinds of ferrite cores, is not possible with tape wound cores at present.
When amorphous or nanocrystalline alloys are employed, the permeability of the magnetic core, and thus the basis for the inductivity L of the choke, are adjusted using, for example, thermal field treatment. When a production batch undergoes uniform thermal treatment, the best degree of variation achievable in magnetic core permeability in relation to the target permeability is ±10%. An additional tolerance of up to ±15% over the target permeability of the magnetic core can result from variations in the permeability of the raw materials, in particular in the composition of the alloy, or can be caused by various technological influences such as the thickness of the tape, the winding tension, geometric irregularities of the tape, magnetostriction, the effects of the glue used to attach the magnetic core in its housing, etc. It is therefore desirable to provide inductive components having a very small inductivity tolerance.